The term "superconductivity" is applied to the phenomenon of immeasurably low electrical resistance exhibited by materials. Until recently superconductivity had been reproducibly demonstrated only at temperatures near absolute zero. As a material capable of exhibiting superconductivity is cooled, a temperature is reached at which resistivity decreases (conductivity increases) markedly as a function of further decrease in temperature. This is referred to as the superconducting transition temperature or, in the context of superconductivity investigations, simply as the critical temperature (T.sub.c). T.sub.c provides a conveniently identified and generally accepted reference point for marking the onset of superconductivity and providing temperature rankings of superconductivity in differing materials.
It has been recently recognized that certain rare earth alkaline earth cuprates as well as other mixed metal cuprates exhibit superconducting transition temperatures (T.sub.c) well in excess of the highest T.sub.c previously known for other metal oxides (a T.sub.c of 13.7.degree. K. reported for lithium titanium oxide). These rare earth alkaline earth copper oxides also exhibit superconducting transition temperatures well in excess of the highest previously accepted reproducible T.sub.c, 23.3.degree. K. for the metal Nb.sub.3 Ge.
Recent discoveries of higher superconducting transition temperatures in rare earth alkaline earth copper oxides are reported in the following publications:
P-1 J. G. Bednorz and K. A. Muller, "Possible High T.sub.c Superconductivity in the Ba-La-Cu-O System", Z. Phys. B.--Condensed Matter, Vol. 64, pp. 189-193 (1986) revealed that polycrystalline compositions of the formula Ba.sub.x La.sub.5-x Cu.sub.5 O.sub.5(3-y), where x=1 and 0.75 and y&gt;0 exhibited superconducting transition temperatures in the 30.degree. K. range:
P-2 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, and Y. Q. Wang, "Evidence for Superconductivity above 40K in the La-Ba-Cu-O Compound System", Physical Review Letters, Vol. 53, No. 4, pp. 405-407, Jan. 1987, reported increasing T.sub.c to 40.2.degree. K. at a pressure of 13 kbar. At the end of this article it is stated that M. K. Wu increased T.sub.c to 42.degree. K. at ambient pressure by replacing Ba with Sr.
P-3 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, and Z. J. Huang, "Superconductivity at 52.5K in the Lanthanum-Barium-Copper-Oxide System", Science Reports, Vol. 235, pp. 567-569, Jan. 1987, a T.sub.c of 52.5.degree. K. for (La.sub.0.9 Ba.sub.0.1).sub.2 CuO.sub.4-y at high pressures.
P-4 R. J. Cava, R. B. vanDover, B. Batlog, and E. A. Rietman, "Bulk Superconductivity at 36K in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 ", Physical Review Letters, Vol. 58, No. 4, pp. 408-410, Jan. 1987, reported resistivity and magnetic susceptibility measurements in La.sub.2-x Sr.sub.x CuO.sub.4, with a T.sub.c at 36.2.degree. K. when x=0.2.
P-5 J. M. Tarascon, L. H. Greene, W. R. McKinnon, G. W. Hull, and T. H. Geballe, "Superconductivity at 40K in the Oxygen-Defect Perovskites La.sub.2-x Sr.sub.x CuO.sub.4-y ", Science Reports, Vol. 235, pp. 1373-1376, Mar. 13, 1987, reported title compounds (0.05.ltoreq.x.ltoreq.1.1) with a maximum T.sub.c of 39.3.degree. K.
P-6 M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, "Superconductivity at 93K in a New Mixed-Phase Y-Ba-Cu-O Compound System at Ambient Pressure", Physical Review Letters, Vol. 58, No. 9, pp. 908-910, Mar. 2, 1987, reported stable and reproducible superconducting transition temperatures between 80.degree. and 93.degree. K. at ambient pressures for materials generically represented by the formula (L.sub.1-x M.sub.x).sub.a A.sub.b D.sub.y, where L=Y, M=Ba, A=Cu, D=O, x=0.4, a=2, b=1, and y.ltoreq.4.
The experimental details provided in publications P-1 through P-6 indicate that the rare earth alkaline earth copper oxides prepared and investigated were in the form of cylindrical pellets produced by forming an intermediate oxide by firing, grinding or otherwise pulverizing the intermediate oxide, compressing the particulate intermediate oxide formed into cylindrical pellets, and then sintering to produce a polycrystalline pellet. The pellets may then be ground to form powders of the superconducting ceramics. Alternative methods for forming sintered powders are also applicable to the formation of the superconducting ceramic powders.
In many of the applications for use of these powders, such as in the preparation of thick film circuits disclosed in the copending patent application of Strom et al, U.S. Ser. No. 068,391 filed July 1, 1987 entitled "Conductive Thick Films and Processes for Film Preparation", commonly assigned and here incorporated by reference now U.S. Pat. No. 4,908,346, the powders may be dispersed in aqueous media to produce screen printable inks. Also, powders may often be stored for extended periods of time where they are subject to exposure to atmospheric moisture.
Many of these superconducting ceramic compounds, e.g. perovskite oxides such as YBa.sub.2 Cu.sub.3 O.sub.7-x, are unstable in the presence of moisture at room temperature, particularly when they are in powdered form thereby exposing a large reactive surface area. The reaction with moisture and possibly carbon dioxide produces a complex multiphase system and often results in a loss or deterioration of the superconductive properties. The moisture induced degradation of these compounds needs to be prevented in order to extend the shelf life, processing parameters and longevity in service of these materials and the articles made therefrom and also to permit flexibility of using aqueous-based solvents to make superconducting pastes or inks useful in the manufacture of thick film devices.
Polyester ionomers constitute an art recognized class of polymers. The following patents are illustrative of varied forms of these polymers:
P-7 Noonan et al U.S. Pat. No. 4,097,282; PA0 P-8 Merrill et al U.S. Pat. No. 4,252,921; PA0 P-10 Noonan et al U.S. Pat. No. 4,291,153; PA0 P-11 Noonan et al U.S. Pat. No. 4,395,475; and PA0 P-12 Noonan et al U.S. Pat. No. 4,419,437.